Many processes and catalysts are known for the preparation of homopolymeric or copolymeric olefins and other polymers. Ziegler-Natta catalyst compositions, developed in the 1950s, were found to be particularly useful in the preparation of polyolefins. These catalyst compositions comprise transition metal compounds such as titanium tetrachloride and an alkylaluminum (e.g., triethylaluminum) cocatalyst. The systems were found to be advantageous because of their high activity, and were largely consumed during polymerization.
More recent catalyst systems for use in preparing polyolefins and other polymers are "metallocenes." The term "metallocene" was initially coined in the early 1950s to refer to dicyclopentadienyliron, or "ferrocene," a structure in which an iron atom is contained between and associated with two parallel cyclopentadienyl groups. In general, the term is now used to refer to organometallic complexes in which a metal atom (not necessarily iron) is coordinated to at least one cyclopentadienyl ring ligand.
In contrast to the traditional Ziegler-Natta catalysts, metallocenes can provide a polymer composition containing a plurality of polymer molecules of substantially the same molecular structure. That is, if one high purity metallocene catalyst is used, the variance in the composition or molecular weight of the individual polymer molecules produced is minimal. With metallocenes, then, it is possible to control compositional distribution and other aspects of polymer molecular structure with unprecedented precision. Metallocene catalysts have other advantages as well. For example, metallocenes: (a) can polymerize almost any vinyl monomer irrespective of molecular weight or steric considerations; (b) provide the ability to control vinyl unsaturation in the polymers produced; (c) enable polymerization of .alpha.-olefins with very high stereoregularity to give isotactic or syndiotactic polymers; and (d) can function as hydrogenation catalysts for polymers as well as monomers. A. D. Horton, "Metallocene Catalysis: Polymers by Design," Trends Polym. Sci. 2(5):158-166 (1994), provides an overview of metallocene catalysts and their advantages, and focuses on now-conventional complexes of Group IV transition metal complexes and cyclopentadienyl ligands (Cp.sub.2 MX.sub.2, wherein Cp represents a cyclopentadienyl ligand, M is Zr, Hf or Ti, and X is Cl or CH.sub.3).
Metallocenes have also been found to be useful in catalyzing other types of reactions, i.e., reactions other than polymerization reactions. For example, metallocenes have been used as hydrogenation catalysts, dehydrocoupling catalysts, cyclization catalysts, substitution reaction catalysts, hydroformylation catalysts, carbomagnesation catalysts and hydrosilylation catalysts. See, e.g., Lu et al. (1997), Lanzhou Inst. Chem. Phys. 11(6):476-483; Halterman (1992), "Synthesis and Applications of Chiral Cyclopentadienyl Complexes," Chem. Rev. 92:965-994; and Hoveyda et al. (1996), "Enantioselective C--C and C--H Bond Formation Mediated or Catalyzed by Chiral ebthi Complexes of Titanium and Zirconium," Angew. Chem. 35:1262-1284. Thus, metallocenes are extremely versatile and valuable catalysts. However, prior metallocene catalysts have proved to be relatively difficult and time-consuming to synthesize, requiring expensive equipment, extreme reaction conditions, and multi-step processes that ultimately result in a low yield of the desired product. Accordingly, there is a need in the art for new metallocene catalysts that can be synthesized without any of the aforementioned problems. That is, it would be desirable to have a simple, straightforward method for preparing chiral metallocenes that can be used in stereospecific catalysis, to be used in the stereospecific polymerization of olefins as well as in other stereospecific bond formation reactions. The present invention is addressed to the aforementioned needs in the art.